A much-anticipated clinical trial of two drugs used as smoking cessation therapies has concluded that they do not cause neuropsychiatric side effects such as suicidal ideation.

The EAGLES study - published in The Lancet - backed the safety and efficacy of Pfizer's Chantix/Champix (varenicline) and GlaxoSmithKline's Zyban (bupropion) as well as nicotine replacement therapies in helping people quit smoking.

Questions about the safety of Champix and Zyban first came to the fore in 2009, when the FDA ordered Pfizer and GSK to include box warnings on their products to warn of their possible link to “changes in behaviour, depressed mood, hostility and suicidal thoughts”.

The label change was prompted by an analysis of adverse event reports (AERs), although at the time the FDA noted that some of these effects might be attributed to nicotine withdrawal.

The researchers behind the 8,100-patient EAGLES trial found there was no significant increase in serious neuropsychiatric adverse events with Champix or Zyban compared to treatment with either a placebo or nicotine patch in people with or without a history of psychiatric disorder.

They also found that smokers treated with Champix had significantly higher quit rates than those treated with Zyban, nicotine patch or placebo. Patients treated with each of the medications had higher abstinence rates than those treated with placebo - 22% of Champix-treated people were abstinent at six months, compared to 16% of the Zyban and nicotine patch groups and 9% of those on placebo.

Robert West of University College London - one of the authors of the paper - said the results “should reassure regulatory authorities, doctors and patients about the safety and effectiveness of medicines to help smokers to stop, whether or not those smokers have a history of a psychiatric disorder”.

“Every smoker should receive the offer of evidence-based support to stop at least once a year - currently most do not,” he noted.

For Pfizer, the results provide an opportunity for Champix to start building sales momentum that stalled once the FDA made its concerns about safety known. The company has indicated it will discuss the possibility of removing the box warning with the US regulator.

Once tipped as a $1bn-plus blockbuster, Champix has failed to meet initial expectations although sales - which were $671m last year down from a peak of nearly $850m in 2008 - have nevertheless made it a useful mid-range product for Pfizer.

The controversy over the drug's safety has cost the US drugmaker in more than lost sales, however. In 2013 the company paid out in the region of $300m to settle around 2,900 lawsuits brought by people claiming to have been injured while taking Champix.